Method of degassing steel



Feb. 28, 1967 T. MESSING 3,306,731

METHOD OF DEGASSING STEEL Filed Nov. 18, 1964 United States Patent 0 i 3,306,731 METHOD OF DEGASSING STEEL Theodor Messing, Mulheim (Ruhr), Speldorf, Germany,

assignor to Standard-Messo Gesellschaft fiir Chemictechnik m.h.H. & (10., Duisburg, Germany Filed Nov. 18, 1964, Ser. No. 412,206 Claims priority, application Germany, Nov. 20, 1963, St 21,343 2 Claims. (til. 75-49) The degasification of steel in vacuum has become an important method in steel plants. According to this method, advantage is taken of the fact that the gas content of steel, especially the content in hydrogen, oxygen and nitrogen, depends on the pressure. Under normal conditions, steel starts to give off or yield gas at a pressure of from 300 to 100 millimeter mercury column absolute. The reduction in pressure for causing molten steel to give off gas does not, however, ordinarily yield a degasification corresponding to the pressure, because the principles of diffusion principally determine the quantity of gas which is to be given off during the period during which the vacuum treatment takes place. Accordingly, the yield of gas at a certain predetermined vacuum is dependent primarily on the time period of treatment and on the length of the path over which the gas has to move within the steel melt up to the steel surface which yields the gas.

A number of methods have been developed which are intended to produce the necessary high vacuum in order to obtain the most favorable gas content and to extend the time period during which the high vacuum is effective as long as possible while making the diffusion path as short as possible.

These methods have furthermore been developed also with regard to the most favorable possibility of application for the respective prevailing conditions, and as a particular field of application has been considered the production of heavy forge blocks and the teeming of steel into ingot molds in a vacuum. According to this method, the molten steel is poured into an intermediate bottom pour ladle which is placed on a vessel in which a vacuum prevails. The steel flows through the bottompour opening of said intermediate ladle into an ingot mold arranged in a vacuum-tight container. With this method, it is necessary to evacuate the vacuum chamber prior to using the same, the degree of vacuum depending on the limiting conditions for the respective problem involved. In addition thereto, it is necessary to pour the steel into the intermediate ladle in such a way that the bottom-pour opening will always be covered by the outflowing steel so that atmospheric air will be prevented from entering the vacuum container. To this end, the outflow from the intermediate ladle and the inflow to the pouring ladle are so adjusted with regard to each other that always a quantity of liquid steel remains in the intermediate ladle.

When the steel passes through the bottom-pour opening and enters the vacuum chamber, a sudden pressure drop from atmospheric pressure to the respective vacuum occurs as a result of which the steel is more or less torn apart in conformity with the gas content and the respective vacuum, and the thus torn apart steel drops into the ingot mold in the form of a rain of droplets. While the rain of droplets is on its way into and is in the ingot mold, a degasification of the steel occurs. The diffusion paths of the gas in the steel are determined by the magnitude of the droplets.

It has now been found that depending on the steel alloy and the steel pre-treatment, the gas content of the steel and thus the quantity of gas'to be removed from the steel may vary greatly. This means that an uncon- 3,366,73l Patented Feb. 28, 1967 trolled expansion of the steel from the intermediate ladle into the vacuum chamber will bring about an improvement in quality of the product to be cast, for. instance cast blocks, only when the degasification of the steel is not too high. The quality of the product will with too great a yield of gas from the steel .become poor in view of the fact that the steel is torn into finest droplets which produce a wide spray jet and bring about that many droplets are sprayed against the ingot mold wall prior to the liquid steel rising in the mold reaching the respective wall sections. Due to the cooling effect exerted by the cold ingot mold wall upon the steel droplets, the latter solidify and are contacted by the rising liquid steel only after a relatively long period of time so that following the solidification of the block, a poor surface will be obtained. Moreover, it may also happen that these solidified droplets which collect in form of conglomerates get detached from the wall and drop back into the melt. Inasmuch as also material exchange of the solidified droplets with the wall of the ingot mold is possible, it Will be appreciated that the embedded conglomerates may cause impurifications in the cast block. It should also be noted that a droplet rain which represents a particularly strong spray brings about that the steel sprays out of the ingot mold and thus causes losses or beard-shaped extensions at the upper edges of the ingot mold. As a result thereof, particularly with strong spray effect and following the cooling off of the block, the block is suspended in the ingot mold and a considerable danger of tears will ensue.

The above effects could be ascertained to a full degree only after it had become possible in recent times by emplyong increased pump outputs according to the steam jet principle, to produce high casting outputs with steel of any random high gas content at extremely low pressure. In installations working without increased pump output, the above-mentioned features and phenomena could not be observed in view of the limited suction of mechanical pumps.

In order to avoid the drawbacks inherent to the employment of low pressures, it has been customary to select the magnitude of the vacuum in conformity with the prevailing conditions, in particular, the spray effect of the jet. To this end, the vacuum was reduced and instead of obtaining a good final gas content by selecting a more favorable vacuum, the degasification of the steel actually became poorer.

It is, therefore, an object of the present invention to provide a method of and an apparatus for post-degasifying molten steel, which will overcome the above-mentioned drawbacks.

It is another object of this invention to provide a method of and an apparatus for post-degasifying molten metals in a vacuum container, which will make it possible to take full advantage of the low pressures in the vacuum container as they are obtained by increased pump outputs, without encountering a drop in quality of the product in view of too high a pressure drop between the ladle and the vacuum chamber.

It is still another object of this invention to provide a method of and an apparatus as set forth above, which will prevent the metal melt from becoming impurified by metal droplet spray against the walls of the ingot mold.

It is also an object of this invention to provide a method and apparatus as set forth above, which will yield a controlled expansion of the metal melt from the ladle into the vacuum chamber.

These and other objects and advantages of the invention will appear more clearly from the following specification in connection with the accompanying drawing diagrammatically illustrating a section of an apparatus according to the present invention for carrying out a method of this invention.

According to the present invention it is suggested to lower the pressure stepwise from the pressure in an intermediate ladle to the pressure in a vacuum container. To this end, the steel which is poured from the pouring ladle into an intermediate ladle mounted On a vacuum container is for purposes of pre-degasification passed into a chamber from which it is subsequently with a certain predetermined loss in pressure poured in a vacuum of desired magnitude into the ingot mold in a vacuum container to thereby bring about a degasification of the steel. In order to control the pressure drop in the chamber, according to the present invention, an additional gas may be blown into the chamber from the outside.

An apparatus for practicing the method according to the present invention consists primarily of a degasification device known per se for degasifying the pouring jet and comprises an intermediate ladle arranged on the vacuum contatiner for receiving the ingot mold, the intert mediate ladle being supplemented by an additional chamber. This chamber is arranged in the vacuum container above the ingot mold and communicates with the interior of the intermediate ladle through an opening therein. This chamber is lined with refractory material and is connected to the container lid preferably by holding means.

More specifically, referring to the drawing, the drawing shows a degasifying arrangement known per se for degasifying a pouring jet. vacuum container 1 having an ingot mold 2 arranged on support 8 and being closed air-tight by a container cover 5. The arrangement furthermore comprises an intermediate ladle 3 which is mounted above the vacuum container 1. The intermediate ladle 3 is filled with molten steel by means of a main ladle 13. The intermediate ladle 3 communicates with the interior of vacuum container 1 through a bottom-pour opening 11 which is adapted to be closed by a plug 10. The vacuum in the interior of said vacuum container 1 is produced and maintained by suction pumps (not shown) which are connected to the vacuum container 1 through the intervention of a suction pipe 6.

In conformity with the present invention, the vacuum container 1 is above the ingot mold 2 provided with an additional chamber 4 which communicates with the intermediate ladle 3 through the bottom-pour opening 11 of said intermediate ladle. Chamber 4 is by holding means 7 connected to container cover and has a discharge opening 12 which is greater in diameter than the bottom-pour opening 11. Furthermore, a gas pipe 14 leads into chamber 4 in order to permit the supply of inert gas from the outside into chamber 4 for purpose of making up for the loss in pressure therein.

The purpose of chamber 4 consists in receiving the steel which passes from the intermediate ladle 3 into vacuum container 1 and also to permit this steel to pass into ladle 2. The dimensions of the cross sections through which the steel passes are so selected that when the steel passes through the respective openings, a certain preselected resistance will be encountered and thus a certain preselected drop in pressure. In order to assure a proper free flow, it is necessary that the discharge opening 12 of chamber 4 is larger than the bottom-pour opening 11. In this connection, however, care has to be taken that the discharge opening 12 of chamber 4 be selected so small that the flowing steel including the pre-degasified gas ingredients will be able to produce a pressure drop in conformity with the well known laws governing the flow of liquids.

Inasmuch as the theoretical calculations of the respective conditions lead only approximately to the desired effect, it is suggested, according to the present invention, to precisely maintain the pressure reduction, which corresponds to the respective conditions of operation, by blowing inert gas from the outside into chamber 4. The

This arrangement comprises a additional flow of said inert gas will be able to vary the said resistance and thus the pressure to the desired extent. A favorable result of the method according to the present invention with an apparatus according to the invention was obtained at a pressure in chamber 4 which was selected within a pressure range corresponding to the es= sential degasification of oxygen or CO reaction. Depend ing on the composition of the steel, this pressure range is between 50 and 30 millimeter mercury column absolute.

Operation-According to the method of the present invention the steel is poured from main ladle 13 into intermediate ladle 3 from which, after plug 10 has been opened, it passes through the bottom-pour opening 11 into vacuum container 1. In vacuum container 1 the steel first enters the auxiliary chamber 4 in which only at the start of the pouring operation the vacuum will be the same as in the ingot mold 2 and in vacuum container 1. While the discharge opening 12 of chamber 4 is larger than the bottom-pour opening 11 so that the steel can pass from chamber 3 through chamber 4 without backing up, opening 12 is still small enough to produce a loss in pressure.

The molten steel will in this way when entering chain her 4 be sprayed in form of droplets and will be degasi= fied, however, the said droplets will be collected again and will leave chamber 4 in the form of a clustered jet. In view of the fiow resistance of the steel gas mixture passing into the higher vacuum from chamber 4, an own pressure will prevail in chamber 4 with regard to the higher vacuum, and this overpressure may be controlled for instance by means of a quantity of foreign gas. The partially degasified steel is now in the higher vacuum sprayed considerably less far whereby disturbing phe= nomena which would interfere with an optimum degasi= fication will be avoided.

Experience has shown that at a pressure of from 0.5 to 5 torrs in the vacuum container 1, a pressure of from S0 to 250 torrs in chamber 4 yields a favorable result.

The method according to the present invention yields the known advantages of the degasification of pouring jets and the favorable effect of a high vacuum without the necessity of considering the gas content of the steel and the pouring speed. The low pressure in the vacuum container is produced by steam jet vacuum pumps of high suction output and is maintained during all phases of the vacuum treatment.

In view of the stepwise degasification of the molten steel in a vacuum container according to the present in vention, the atomizing effect of the outfiowing steel is avoided. Inasmuch as the steel leaves chamber 4 for vacuum container 1 in a pregasified condition, it will be possible in vacuum container 1 to employ a higher vacu= um and thereby to obtain a more favorable degasification of the steel. Furthermore, the degasifying effect according to the present invention is aided by the fact that in view of chamber 4, depending on the liquid content of this chamber, the total dropping time of the steel is increased whereby the diffusion is favorably affected.

It is, of course, to be understood, that the present invention is, by no means, limited to the above-outlined method and arrangement, but also comprises any modifications within the scope of the appended claims. Thus, instead of the illustrated ingot mold 2, there may also be provided in the vacuum container a ladle equipped with a closure.

The manner in which the method is carried out will be demonstrated by the following examples, the first of which concerns a completely killed steel while the other one refers to a semi-killed steel.

Example N0. 1

The steel was molten in an electric arc furnace by the two-slag practice, the steel being completely killed. Its composition was as follows: 035% C; 0.26% Si; 0.59% Mn; 1.14% Cr; 0.21% Mo.

An amount of 80 tons of steel was tapped which was intended for casting a vacuum block of 70 tons.

The diameter of the discharge opening of the intermediate ladle 11 was 40 mm.; that of the discharge opening of the'chamber 12 was 45 mm. Depending on the temperature and the composition of the steel, the increase of discharge opening diameters during operation will differ in size, a fact which should be considered when determining those diameters. The vacuum at the beginning of the pouring operation was 0.1 millimeter mercury in both the vacuum chamber 4 and in the chamber 12. During the pouring operation the following average vacuum values were ascertained:

Millimeters mercury In the vacuum chamber 1 (approx.) 0.4 In the chamber 4 (approx.) 80

of oxygen were ascertained.

Example N0. 2

Analysis: 0.33% C; 0.09% Si; 0.66% Mn; 1.20% Cr; 0.18% M0.

Vacuum block tons 70 Diameter of discharge opening of the intermediate ladle mm 40 Diameter of discharge opening of the chamber mm 50 During the vacuum treatment the following pressures were ascertained.

Millimeters mercury In the vacuum chamber 0.5 In the chamber 250 It was not necessary to introduce argon into the chamber 4. The pouring jet showed increased divergence as compared with Test No. 1.

After pouring, 1.2 p.p.m. of hydrogen and 35 p.p.m. of oxygen were found.

What I claim is:

1. A method of stepwise degasifying molten metal, especially molten steel, by pouring the metal to be degasified successively into a first chamber for pre-degasification in a vacuum and from said first chamber into a second chamber for post-degasification in a vacuum, which includes the steps of: introducing the molten metal into said first chamber so that the stream of metal will be torn apart into droplets while maintaining the vacuum in said first chamber lower than that in said second chamber and within desired limits by introducing a gas from the outside into said first chamber, and subsequently passing the thus pre-degasified metal in a continuous and coherent stream into said second chamber for post-degasification at the higher vacuum prevailing therein.

2. A method according to claim 1, which includes maintaining the pressure in said first chamber within the range of from to 250 torrs while maintaining the pressure in said second chamber within the range of from 0.5 to 5 torrs.

Kopineck et al.: Article in Stahl and Eisen, June 21, 1962, vol. 82, No. 13, pp. 852-855.

DAVID L. RECK, Primary Examiner.

HYLAND BIZOT, H. W. TARRING,

Assistant Examiners. 

1. A METHOD OF STEPWISE DEGASIFYING MOLTEN METAL, ESPECIALLY MOLTEN STEEL, BY POURING THE METAL TO BE DEGASIFIED SUCCESSIVELY INTO A FIRST CHAMBER FOR PRE-DEGASIFICATION IN A VACUUM AND FROM SAID FIRST CHAMBER INTO A SECOND CHAMBER FOR POST-DEGASIFICATION IN A VACUUM, WHICH INCLUDES THE STEPS OF: INTRODUCING THE MOLTEN METAL INTO SAID FIRST CHAMBER SO THAT THE STREAM OF METAL WILL BE TORN APART INTO DROPLETS WHILE MAINTAINING THE VACUUM IN SAID FIRST CHAMBER LOWER THAN THAT IN SAID SECOND CHAMBER AND WITHIN DESIRED LIMITS BY INTRODUCING A GAS FROM THE OUTSIDE INTO SAID FIRST CHAMBER, AND SUBSEQUENTLY PASSING THE THUS PRE-DEGASIFIED METAL IN A CONTINUOUS AND COHERENT STREAM INTO SAID SECOND CHAMBER FOR POST-DEGASIFICATION AT THE HIGHER VACUUM PREVAILING THEREIN. 